U.S. patent application number 14/428591 was filed with the patent office on 2015-07-30 for pseudo-absolute position sensing algorithm.
The applicant listed for this patent is EATON CORPORATION. Invention is credited to Derek Paul Dougherty, Andrew Thompson, John Mendenhall White.
Application Number | 20150211892 14/428591 |
Document ID | / |
Family ID | 48096180 |
Filed Date | 2015-07-30 |
United States Patent
Application |
20150211892 |
Kind Code |
A1 |
Dougherty; Derek Paul ; et
al. |
July 30, 2015 |
PSEUDO-ABSOLUTE POSITION SENSING ALGORITHM
Abstract
A system having a position sensing algorithm for determining a
position of an electro-mechanical actuator (EMA) stroke includes a
first rotary component supported for rotation about a first axis,
and a second rotary component supported for rotation about a second
axis. A first rotary encoder may be configured to generate an
output based on an angular position of the first rotary component,
and a second rotary encoder may be configured to generate an output
based on an angular position of the second rotary component. The
first and second rotary components may define a ratio such that the
first and second rotary encoders generate unique combinations of
outputs for an entire stroke of an EMA. A decoder may be provided
having a position sensing algorithm that determines a position of
the EMA stroke based on the ratio between first and second rotary
components and outputs from first and second encoders.
Inventors: |
Dougherty; Derek Paul;
(Muskegon, MI) ; White; John Mendenhall;
(Hudsonville, MI) ; Thompson; Andrew; (Ada,
MI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
EATON CORPORATION |
Cleveland |
OH |
US |
|
|
Family ID: |
48096180 |
Appl. No.: |
14/428591 |
Filed: |
March 14, 2013 |
PCT Filed: |
March 14, 2013 |
PCT NO: |
PCT/US2013/031435 |
371 Date: |
March 16, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61704057 |
Sep 21, 2012 |
|
|
|
Current U.S.
Class: |
702/151 ;
74/640 |
Current CPC
Class: |
F16H 35/16 20130101;
G01D 5/14 20130101; Y10T 74/19 20150115; G01D 5/04 20130101 |
International
Class: |
G01D 5/14 20060101
G01D005/14; F16H 35/16 20060101 F16H035/16 |
Claims
1. A system with an output stage and rotational component,
comprising: a first rotary component and a second rotary component;
a first rotary encoder configured to generate a first output based
on an angular position of the first rotary component; a second
rotary encoder configured to generate a second output based on an
angular position of the second rotary component; and a decoder
including a position sensing algorithm configured to determine a
position of a final output stage using the first and second
outputs, wherein a ratio between the first and the second rotary
components is a non-integer ratio with a relatively prime
decimation, the angular position of the second rotary components
changes an incremental amount for each full rotation of the first
rotary component, and combinations of the first and second outputs
are unique during a select number of rotations of said rotational
component of said output stage.
2. The system of claim 1, wherein the ratio satisfies the equation:
1 > n 1 - y 1 ( gr ) n 1 r 2 ##EQU00009## where y1 (gr) is a
sensor output when the numerical value of the ratio revolutions has
occurred, n1 is a number of bits for the first encoder, and r2 is a
number of bits for the second encoder.
3. The system of claim 2, wherein the ratio is m/n, and m and n are
co-prime.
4. The system of claim 3, wherein the first and second rotary
components comprise gears, and the number of teeth on the first
rotary component is greater than a number of rotations of that
components for an entire mechanical stroke.
5. The system of claim 1, wherein the first rotary component
comprises a motor pinion and the second rotary component comprises
an output gear in an electro-mechanical actuator.
6. The system of claim 1, wherein the first rotary component is
provided in a third gear pass and the second rotary component is
provided in a fourth gear pass in an electro-mechanical
actuator.
7. The system of claim 6, wherein the third gear pass has a gear
ratio of 66/19 and the fourth gear pass has a gear ratio of
60/19.
8. The system of claim 1, wherein the first rotary component is
provided in a second gear pass and the second rotary component is
provided in a fourth gear pass in an electro-mechanical
actuator.
9. The system of claim 8, wherein the second gear pass has a gear
ratio of 63/15 and the fourth gear pass has a gear ratio of
60/19.
10. A system for determining the position of an electro-mechanical
actuator (EMA) stroke, the system comprising: a first rotary
component supported for rotation about a first axis and a second
rotary component supported for rotation about a second axis; a
first rotary encoder configured to generate an output based on an
angular position of the first rotary component and a second rotary
encoder configured to generate an output based on an angular
position of the second rotary component, wherein the first and
second rotary components define a ratio such that the first and
second rotary encoders generate unique combinations of outputs for
an entire stroke of said EMA; and a decoder having a position
sensing algorithm configured to determine a position of said EMA
stroke based on the ratio between the first and second rotary
components and the outputs from the first and second encoders.
11. The system of claim 10, wherein the ratio between the first and
the second rotary components is a non-integer ratio with a
relatively prime decimation
12. The system of claim 10, wherein the angular position of the
second rotary component changes an incremental amount for each full
rotation of the first rotary component.
13. The system of claim 10, wherein the ratio satisfies the
equation: 1 > n 1 - y 1 ( gr ) n 1 r 2 ##EQU00010## where y1
(gr) is a sensor output when the numerical value of the ratio
revolutions has occurred, n1 is a number of bits for the first
encoder, and r2 is a number of bits for the second encoder.
14. The system of claim 13, wherein the ratio is m/n, and m and n
are co-prime.
15. The system of claim 14, wherein the first and second rotary
components are gears, and the number of teeth on the first rotary
component is greater than a number of rotations of that components
for an entire mechanical stroke of said EMA.
16. The system of claim 10, wherein the first rotary component is a
motor pinion and the second rotary component is an output gear of
the EMA.
17. The system of claim 10, wherein the first rotary component is
provided in a third gear pass and the second rotary component is
provided in a fourth gear pass of said EMA.
18. The system of claim 17, wherein the third gear pass has a gear
ratio of 66/19 and the fourth gear pass has a gear ratio of
60/19.
19. The system of claim 10, wherein the first rotary component is
provided in a second gear pass and the second rotary component is
provided in a fourth gear pass of said EMA.
20. The system of claim 19, wherein the second gear pass has a gear
ratio of 63/15 and the fourth gear pass has a gear ratio of 60/19.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a national stage filing based upon
International Application No. PCT/US2013/031435, with an
international filing date of Mar. 14, 2013, which claims the
benefit of U.S. Provisional Application Ser. No. 61/704,057, filed
Sep. 21, 2012, the disclosures of which are incorporated herein by
reference in their entirety.
TECHNICAL FIELD
[0002] The present disclosure relates to position sensing
algorithms, including a position sensing algorithm that uses rotary
position sensors to calculate a positional output of a mechanical
system.
BACKGROUND
[0003] It is known to measure the angular displacement of a shaft
using a rotary variable differential transformer (RVDT). In
general, an RVDT is an electrical transformer that includes a rotor
supported for rotation within a housed stator assembly. The RVDT
provides an output voltage that is linearly proportional to the
angular displacement of the shaft. However, when used to determine
the absolute position of an electro-mechanical actuator (EMA), a
RVDT system typically uses an anti-backlash gear train related to
an output axis of the EMA to determine the absolute position. An
RVDT system may also require drive electronics for excitation and
position decoding. Further, an RVDT system can be relatively
complex, heavy, and costly.
SUMMARY
[0004] An embodiment of a system has a position sensing algorithm
that employs rotary position sensors to determine the position of
an electro-mechanical actuator (EMA) stroke. The system may include
a first rotary component supported for rotation about a first axis
and a second rotary component supported for rotation about a second
axis. A first rotary encoder may be configured to generate an
output based on an angular position of the first rotary component
and a second rotary encoder may be configured to generate an output
based on an angular position of the second rotary component. The
first and second rotary components may define a ratio such that the
first and second rotary encoders generate unique combinations of
outputs for an entire stroke of said EMA. A decoder may be provided
that includes a position sensing algorithm for determining a
position of an EMA stroke based on a ratio between first and second
rotary components and outputs from first and second encoders.
[0005] Various aspects of the present disclosure will become
apparent to those skilled in the art from the following detailed
description of the embodiments, when read in light of the
accompanying drawings.
DESCRIPTION OF THE DRAWINGS
[0006] Embodiments of the invention will now be described, by way
of example, with reference to the accompanying drawing,
wherein:
[0007] FIG. 1 is a diagram of an electro-mechanical actuator (EMA)
having a gear train in accordance with an embodiment of the present
disclosure.
DETAILED DESCRIPTION
[0008] A pseudo-absolute position sensing algorithm according to
one aspect of the teachings use two rotary position sensors to
calculate a positional output of a mechanical system using a
mechanical ratio between two or more axes of rotation. In one
example, the algorithm uses the gear ratio relationship to
calculate the position of a final output stage of an
electromechanical system, which can be any system that uses a gear
train to translate a rotary input into a mechanical output (e.g., a
linear output, a rotary output, etc.). Once the system has been
calibrated and as long as the mechanical relationship of the
sensors is held constant, the algorithm can calculate the current
position.
[0009] FIG. 1 illustrates one possible electro-mechanical actuator
(EMA) system, indicated generally at 10. The EMA system 10 may have
a gear train with at least a first gear, a first absolute rotary
encoder that senses the angular position of the first gear, a
second gear, and a second absolute rotary encoder that senses the
angular position of the second gear. The EMA gear train 10 shown in
the drawings is for illustrative purposes only; those of ordinary
skill in the art will recognize that the sensing algorithm
described herein can be used in any system having a gear train, and
more broadly having a gear ratio, without departing from the scope
of the teachings.
[0010] The first and second encoders can be disposed anywhere in
the EMA system 10. In one example, the first encoder may be mounted
on a motor pinion in the EMA system 10 and the second encoder may
be mounted near an output stage of the EMA system 10. The inputs of
the first and second encoders depends on the angular positions of
the first and second gears, respectively, relative to their
rotational axes. The first and second encoders generate first and
second outputs corresponding to the positions of the first and
second gears and send the outputs to a decoder. Note that in
practice, the first and second gears rarely have a zero index or
timing mark and it is very improbable that encoders be perfectly
aligned with each other or the gear train and thus there be likely
be some initial error. Thus, the algorithm may generate an offset
value to zero out the positions of the encoders at an initial
starting position as an calibration step and store this offset
value in the EMA system 10. Afterward, as long as the encoders do
not change position relative to the first and second gears the
algorithm can determine the absolute position of the EMA
output.
[0011] To determine the absolute angular position of a stroke in
the EMA gear system 10, the first and second outputs from the
rotary encoders create unique combinations for an entire stroke of
the EMA system 10. Based on the mathematical relationships between
the angular positions of the two encoders and the gear ratio
between the first and second gears the decoder generates an output
representing the stroke position of the EMA system 10. The unique
combinations from the rotary encoders ensure that the decoder will
not generate an erroneous position output.
[0012] Generating unique encoder output combinations depends on the
mechanical characteristics of the first and second gears. As is
known in the art, for a given gear ratio, the smaller gear will
change its angular position by an incremental amount for each full
rotation of the larger gear. In one aspect of the teachings,
generating unique combinations of angular positions and outputs
involves ensuring that the first and second gears have a
non-integer gear ratio with a relatively prime decimation. More
particularly, there should be enough decimation in the gear ratio
to sustain all the required gear rotations without repeating
angular position combinations for a given gear ratio at any time
during a stroke of the EMA system 10. The gear ratio may be a
rational number represented as m/n, where both m and n are
relatively prime numbers. For example, for a gear ratio of 157:1,
the gear shaft would rotate 157 times before the angular position
combination would repeat. This would be acceptable if fewer than
157 total gear shaft revolutions were needed to complete a full EMA
stroke. However, if more gear shaft revolutions are needed in the
stroke of the EMA system 10, the gear ratio could be changed to
accommodate this. For example, a gear ratio of 157.2:1 would repeat
after 786 rotations (because 1572 is divisible by 2). However, a
gear ratio of 157.1:1 would require 1571 rotations of the gear
shaft before the angular position combination repeats.
[0013] The algorithm will now be described via two theories:
angular theory and digital theory. In the angular theory, the
algorithm maintains the following relationships through an entire
EMA stroke:
.theta. -- 1 = mod ( x -- 1 , 1 ) 360 ##EQU00001## .theta. -- 2 =
mod ( x -- 2 , 1 ) 360 ##EQU00001.2## .theta. -- 3 = mod ( x -- 1
Gear -- Ratio , 1 ) 360 ##EQU00001.3## .theta. -- 4 = mod ( x -- 2
Gear -- Ratio , 1 ) 360 ##EQU00001.4##
[0014] Where x is the number of gear train revolutions
[0015] .theta..sub.--1.noteq..theta..sub.--4
[0016] .theta..sub.--3.noteq..theta..sub.--4
[0017] For any x.sub.--1 and x.sub.--2.
[0018] As can be seen in the equations above, the functions are
continuous and the unique combinations can be created for a given
gear train provided the relationships above are satisfied. At any
point during the EMA stroke, the gear axis on which the first and
second encoders are mounted generates unique angular position
combinations relative to the axis.
[0019] When the algorithm is used in a digital system, the
resolution of the first and second encoders is limited by the bit
resolution of the encoder output (i.e., the analog-to-digital
conversion of the output). Thus, implementation of the algorithm in
the digital realm needs to take into account the bit resolution and
accuracy to generate the unique angular position combinations. The
relationship described above with respect to the angular theory,
when transferred to the binary realm, may be described as
follows:
y 1 = mod ( x 1 n 1 , n 1 ) ##EQU00002## y 2 = mod ( x 2 n 1 , n 1
) ##EQU00002.2## y 3 = mod ( x 1 n 2 Gear -- Ratio , n 2 )
##EQU00002.3## y 4 = mod ( x 2 n 2 Gear -- Ratio , n 2 )
##EQU00002.4##
[0020] Where x.sub.n is the number of revolutions of sensor 1, and
n1 and n2 are the counts for the individual sensors
n=2.sup.bits.
[0021] y.sub.1.noteq.y.sub.2
[0022] y.sub.3.noteq.y.sub.4
[0023] For any x.sub.1 and x.sub.2.
[0024] In this example, at any point throughout the EMA stroke, the
first and second encoders generate unique binary values
corresponding to the unique angular positions. The resolution of
the first and second encoders is then limited to the angular
degrees per count of the encoder outputs (360 degrees/n).
[0025] Note that the position information from the first and second
encoders may include a large amount of data, making it difficult to
store in a digital memory having limited storage space. To overcome
this issue, the algorithm may translate the outputs of the first
and second encoders into a linear relationship using the gear ratio
and the incremental change in the angular position of the smaller
gear for each full rotation of the larger gear. Based on this
incremental position change, the input of the encoder with the
lowest number of axis revolutions is added to a scaled multiplicand
of the original encoder position based off the current axis's
angular position. Thus, the outputs of the first and second
encoders can be linearly translated as follows:
n 1 = 2 b 1 Total number of counts for sensor 1 n 2 = 2 b 2 Total
number of counts for sensor 2 gr = ( m -- gr x n -- gr x ) ( m --
gr z n -- gr z ) ( m -- gr n m -- g n ) Total gear ratio r 2 = 25
Number of sensor 2 revolutions specified by EMA design r 1 = r 2 gr
Number of sensor 1 revolutions k Uncertainty factor y 1 ( x ) =
floor ( mod ( | x | n 1 , n 1 ) ) Sensor 1 output y 2 ( x ) = floor
( mod ( | x | gr n 2 , n 2 ) ) Sensor 2 output ##EQU00003##
[0026] where
[0027] b1=number of bits for the first encoder
[0028] b2=number of bits for the second encoder
[0029] m_gr.sub.n=numerator of the gear ratio
[0030] n_gr.sub.n=denominator of the gear ratio
[0031] r1=number of rotations in the first encoder required to
complete an entire EMA stroke
[0032] r2=number of rotations in the second encoder required to
complete an entre EMA stroke
[0033] k=uncertainty factor
[0034] floor=rounding down to the nearest integer
[0035] mod=computing the modulo, where mod(x,y) computes the
remainder of dividing x by y (x modulo y)
[0036] The algorithm may also include the following validity check
for the relationships to be valid:
n 1 > k r 2 ##EQU00004## n 2 > k r 1 ##EQU00004.2## 1 > n
1 - y 1 ( gr ) n 1 r 2 ##EQU00004.3##
[0037] The output of the second encoder may be scaled to match the
output of the first encoder as follows:
y 1 -- prime ( x ) = mod ( floor ( y 2 ( x ) gr n 1 n 2 ) , n 1 )
##EQU00005## nraw ( x ) = | y 1 -- prime ( x ) - y 1 ( x ) - ( n 1
- 1 ) if y 1 -- prime ( x ) - y 1 ( x ) - n 1 - 1 y 1 -- prime ( x
) - y 1 ( x ) otherwise ##EQU00005.2##
[0038] The relationship above establishes the difference between
scaled output of the second encoder and the output of the first
encoder. The relationship is made to keep the range less than n1-1
counts to accommodate the limited data storage in the system.
[0039] To shape the difference between the first and second encoder
outputs into a stepwise function based off of nraw, the following
relationship may be applied:
nraw 2 ( x ) = | nraw ( x ) - n 1 if nraw ( x ) n 1 - 1 nraw ( x )
- n 1 if nraw ( x ) -- - 2 nraw ( x ) otherwise ##EQU00006##
[0040] This relationship limits the range of nraw2 to within -2 (an
arbitrary number allowed below zero deviation) to 127.
[0041] The count difference during a gear rotation may be described
as:
w(x)=n1-mod(x n1,n1)
and a stepwise function (with a tolerance of w(g)/2) based off the
gear ratio may be described as:
m ( x ) = floor ( nraw 2 ( x ) - w ( gr ) 2 w ( gr ) )
##EQU00007##
[0042] The linear function y based off the position outputs of the
first and second encoders can be described as:
y(x)=y2(x)+m(x)n
[0043] The above algorithm may also include an additional gear
train constraint:
1 < n 1 - y 1 ( gr ) < n 1 r 2 ##EQU00008##
Where y1 (gr) is an encoder output when the numerical value of the
gear ratio revolutions has occurred. This constraint ensures that
for each revolution of, for example, the first encoder, the output
of the second encoder changes by at least one count, but less than
the number of first encoder counts per scaled revolutions of the
second encoder.
[0044] The system above therefore provides accurate position
sensing via low-cost rotary encoder sensors instead of higher-cost
devices, such as rotary or linear variable differential
transformers. The encoders reduce system setup times and support
because they can be electronically calibrated instead of manually
calibrated, and they also use a much simpler electronic
configuration than existing systems. The system also allows
position sensing of any mechanically-constrained input, such as an
electric motor. Using the gear train characteristics in the sensor
algorithm has the added advantage of being able to detect problems
in the gear train itself (e.g., free play or backlash).
[0045] Although the above example focuses on an actuator system
with a gear train, the algorithm can be used in any system that
contains a mechanical advantage using two or more rotational axes
for determining a total number of axial rotations of an output
stage (which does not have to be the final output of the system)
based on the relationships of the mechanical ratios. In other
words, the algorithm can be used in any system that translates a
rotary output to a mechanical output. Once the system is
calibrated, it can sense the absolute position of the final output
as long as the mechanical relationship of the axes, and therefore
the encoders, is held constant.
[0046] It will be appreciated that the above teachings are merely
exemplary in nature and is not intended to limit the present
teachings, their application or uses. While specific examples have
been described in the specification and illustrated in the
drawings, it will be understood by those of ordinary skill in the
art that various changes may be made and equivalents may be
substituted for elements thereof without departing from the scope
of the present teachings as defined in the claims. Furthermore, the
mixing and matching of features, elements and/or functions between
various examples is expressly contemplated herein so that one of
ordinary skill in the art would appreciate from this disclosure
that features, elements and/or functions of one example may be
incorporated into another example as appropriate, unless described
otherwise, above. Moreover, many modifications may be made to adapt
a particular situation or material to the teachings of the present
disclosure without departing from the essential scope thereof.
Therefore, it is intended that the present teachings not be limited
to the particular examples illustrated by the drawings and
described in the specification as the best mode presently
contemplated for carrying out the teachings of the present
disclosure, but that the scope of the present disclosure will
include any embodiments falling within the foregoing description
and the appended claims.
* * * * *